Fermi Gamma-ray Haze via Dark Matter and Millisecond Pulsars

TL;DR

This paper investigates astrophysical and dark matter sources for the Fermi gamma-ray haze, analyzing models involving dark matter annihilation and millisecond pulsars, and assesses their viability based on gamma-ray observations and astrophysical constraints.

Contribution

It compares dark matter annihilation and millisecond pulsars as sources for the gamma-ray haze, proposing that MSPs could explain the haze if a large population exists and certain conditions are met.

Findings

DM annihilation into W+W- and b-bbar produces significant gamma rays above 10 GeV.

MSPs could account for the haze if 20,000-60,000 are present in the halo.

A boost factor of about 100 is needed for leptonic DM models to match observations.

Abstract

We study possible astrophysical and dark matter (DM) explanations for the Fermi gamma-ray haze in the Milky Way halo. As representatives of various DM models, we consider DM particles annihilating into W+W-, b-bbar, and e+e-. In the first two cases, the prompt gamma-ray emission from DM annihilations is significant or even dominant at E > 10 GeV, while inverse Compton scattering (ICS) from annihilating DM products is insignificant. For the e+e- annihilation mode, we require a boost factor of order 100 to get significant contribution to the gamma-ray haze from ICS photons. Possible astrophysical sources of high energy particles at high latitudes include type Ia supernovae (SNe) and millisecond pulsars (MSPs). Based on our current understanding of Ia SNe rates, they do not contribute significantly to gamma-ray flux in the halo of the Milky Way. As the MSP population in the stellar halo of the Milky Way is not well constrained, MSPs may be a viable source of gamma-rays at high latitudes provided that there are ~ 20 000 - 60 000 of MSPs in the Milky Way stellar halo. In this case, pulsed gamma-ray emission from MSPs can contribute to gamma-rays around few GeV's while the ICS photons from MSP electrons and positrons may be significant at all energies in the gamma-ray haze. The plausibility of such a population of MSPs is discussed. Consistency with the Wilkinson Microwave Anisotropy Probe (WMAP) microwave haze requires that either a significant fraction of MSP spin-down energy is converted into e+e- flux or the DM annihilates predominantly into leptons with a boost factor of order 100.